In cementing casing or liners the procedure typically involves dropping one or more balls for engagement with a downhole seat sized for that ball to allow pressure buildup to set downhole devices such as external casing packers. After the ball is dropped and the downhole equipment is set, the delivery of the cement occurs in conjunction of delivery of one or more wiper plugs or darts down the casing. These plugs separate mud from cement or clean the inside of the casing.
Typically the ball-dropping device is located below the dart-releasing device so that the darts must travel past the ball-releasing device after it has dropped the balls. One problem with this layout is that the ball dropping device, after release of the ball, presents either a large opening or edges that can engage the trailing cups on the dart as it is pumped by. What has happened is that tears can develop in these cups allowing fluid bypass around the dart. This can stop the forward motion of the dart or impede its ability to separate fluids or to clean the inside wall of the casing or tubular as it is forced downhole. Accordingly, as described below with regard to the preferred embodiment, as solution to this problem has been devised to try to minimize the tendency to tear the darts as they pass the ball release device.
In another aspect, a provision is made to prevent the darts from coming back uphole, in the event of a pressure surge. Such darts are retained from traveling above their release mechanism. The release mechanism for the darts features, in the preferred embodiment, individual release barrels for each dart allowing for the darts to be dropped in any order. It further allows observation of what dart is in which barrel without affecting the operation of the other barrels holding other darts. Each barrel is movable between a fully misaligned and fully aligned position with the casing or tubular and can be locked in at least two positions. A handle assembly stays with the dart dropping unit and manipulation of the integrated operating handle acts to defeat the lock and rotate a barrel into an aligned position with the casing for launch of the dart.
U.S. Pat. No. 6,182,752 shows a tool that drops darts by continuing rotation in a fixed direction requiring a predetermined order of dropping once the darts are loaded and no provision for checking which dart is in which barrel after loading.
The above described advantages and other features of the invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the claims, which appear below.